Solution polymerization processes are typically carried out at temperatures that are above the melting point of the ethylene homopolymer or copolymer product. In a typical solution polymerization process, catalyst components, solvent, monomers and hydrogen are fed under pressure to one or more reactors.
For ethylene polymerization, or ethylene copolymerization, reactor temperatures can range from about 80° C. to about 300° C. while pressures generally range from about 3 MPag to about 45 MPag. The ethylene homopolymer or copolymer produced remains dissolved in the solvent under reactor conditions. The residence time of the solvent in the reactor is relatively short, for example, from about 1 second to about 20 minutes. The solution process can be operated under a wide range of process conditions that allow the production of a wide variety of ethylene polymers. Post reactor, the polymerization reaction is quenched to prevent further polymerization, by adding a catalyst deactivator. Optionally, if a heterogeneous catalyst formulation is employed in the third reactor, the deactivated solution is passivated, by adding an acid scavenger. The deactivated solution, or optionally the passivated solution, is then forwarded to polymer recovery where the ethylene homopolymer or copolymer is separated from process solvent, unreacted residual ethylene and unreacted optional α-olefin(s).
There is a need to improve the continuous solution polymerization process, for example, to increase the molecular weight of the ethylene interpolymer produced at a given reactor temperature. Given a specific catalyst formulation, it is well known to those of ordinary experience that polymer molecular weight increases as reactor temperature decreases. However, decreasing reactor temperature can be problematic when the viscosity of the solution becomes too high. As a result, in the solution polymerization process there is a need for catalyst formulations that produce high molecular weight ethylene interpolymers at high reactor temperatures. The catalyst formulations and solution polymerization processes disclosed herein satisfy this need.
In the solution polymerization process there is also a need for catalyst formulations that are very efficient at incorporating one or more α-olefins into a propagating macromolecular chain. In other words, at a given [α-olefin/ethylene] weight ratio in a solution polymerization reactor, there is a need for catalyst formulations that produce lower density ethylene/α-olefin copolymers. Expressed alternatively, there is a need for catalyst formulations that produce an ethylene/α-olefin copolymer, having a specific density, at a lower (α-olefin/ethylene) ratio in the reactor feed. Such catalyst formulations efficiently utilize the available α-olefin and reduce the amount of α-olefin in solution process recycle streams.
The catalyst formulations and solution process disclosed herein, produce unique ethylene interpolymer products that have desirable properties in a variety of end use applications, for example applications that employ ethylene interpolymer films. Non-limiting examples of desirable film properties include higher film stiffness, higher film tensile yield, higher film tear resistance, lower hexane extractables, lower seal initiation temperature and improved hot tack performance. Films prepared from the ethylene interpolymer products, disclosed herein, have these desirable properties.